1. Field of the Invention
The invention relates to a hydrostatic drive for driving various types of vehicles.
2. Discussion of the Prior Art
A hydrostatic drive according to the precharacterising clause of Claim 1 is known from EP 0 547 947 A1. In the case of the hydrostatic drive disclosed in this publication, two vehicle wheels lying opposite on a vehicle axle are driven in each case by two hydraulic motors arranged in pairs on a common shaft. The hydraulic fluid delivered in a working circuit by a hydraulic pump branches upstream of the hydraulic motors arranged in pairs. Whereas the hydraulic fluid from the outlet of one of the two hydraulic motors arranged in pairs flows back directly to the hydraulic pump, the outlet of the other hydraulic motor arranged on the same shaft is connected to the hydraulic pump via a further hydraulic motor in each case, these further hydraulic motors driving vehicle wheels of another vehicle axle. In the case of the hydrostatic drive disclosed in this publication, no measures are provided to prevent a slip occurring at one of the mutually opposite vehicle wheels which considerably reduces the efficiency of the drive.
EP 0 505 254 A1 discloses a hydrostatic drive in which all the hydraulic motors driving different vehicle wheels are connected in parallel to the hydraulic pump. Speed sensors are provided on the output shafts of the individual hydraulic motors. As a function of the speeds determined at the individual output shaft, the amount of pressure fluid flowing through the assigned hydraulic motors can be regulated by adjustable, throttled branch valves, so that possible speed differences are equalised and in particular steering or exact straight-line driving permitted. However, this arrangement has only limited use for equalising a slip at one of the vehicle wheels.
EP 0 378 742 A2 discloses a hydrostatic drive in which a first and second drive train are completely separated from each other on cornering, the first drive train having a first hydraulic pump and a first hydraulic motor and the second drive train having a second hydraulic pump and a second hydraulic motor. In order to permit as exact a straight-line driving as possible, the hydraulic motors can be mechanically connected to each other on the one hand by means of a mechanical coupling on straight-line driving. On the other hand, the separated hydraulic working circuits are hydraulically connected to each other by valves on straight-line driving. A measure for preventing the slip at one of the two drive trains is not disclosed in this publication.
DE-A 20 26 910 discloses the arrangement of a first hydraulic pump, a first hydraulic motor, a second hydraulic pump and a second hydraulic motor in series in a common working circuit. Although in the case of this drive a slip is largely avoided owing to the hydraulic rigid coupling between the two hydraulic motors, the efficiency of this kind of drive is substantially reduced owing to the series arrangement of the two hydraulic motors.
The object on which the invention is based is to provide a hydrostatic drive for driving a plurality of drive trains, in which a slip at one of the drive trains is prevented without substantially reducing the efficiency.
The invention is based on the finding that it is advantageous to provide two hydraulic motors mechanically coupled to each other on each drive train, in each case one of the hydraulic motors being arranged in a working circuit and serving for the direct drive of the assigned drive train, whereas the other two hydraulic motors are hydraulically connected to each other via a secondary circuit. If a slip occurs at the first drive train, the associated hydraulic motor arranged in the secondary circuit works as a pump and generates a braking pressure in the secondary circuit. Since the speed of the hydraulic motor, arranged in the secondary circuit, of the second drive train is limited, the braking pressure built up in the secondary circuit reduces the speed at the first drive train. This avoids a situation where the slip occurring at the first drive train uses an excessively large amount of pressure fluid in the working circuit. The hydraulic power of the working circuit can therefore act undiminished on the second drive train which is not subjected to a slip.
Advantageous developments of the invention are specified in the subclaims.
Advantageously, switching valves are arranged in the secondary circuit in such a way that two of the hydraulic motors arranged in pairs on the drive trains are hydraulically interconnected in the secondary circuit, in the manner described above, only when a slip actually occurs. As long as no slip occurs, these hydraulic motors are, in contrast, connected via the switching valves to the working circuit, so that the output torque increases. The switching valves can be driven, for example, electrically via a control unit which determines the occurrence of a slip for example by a comparison of the drive-train speeds detected by means of speed sensors or by detection of a pressure drop at the hydraulic motors situated in the working circuit.
The hydraulic fluid can be fed into the secondary circuit by direct connection to a feed line via appropriate nonreturn valves. Alternatively, it is possible to use the low pressure of the working circuit as feed pressure for the secondary circuit. The feed is then expediently effected via a suitable switching valve for the pressure change.
The invention is also suitable for three, four or more drive trains. In this case, each drive train has two hydraulic motors, in each case one hydraulic motor being connected to the working circuit and another to the secondary circuit. It is also possible to provide a plurality of secondary circuits. The lines of the secondary circuit can be connected via a throttle, as a result of which a limited slip is allowed between the drive trains and thus the steering of the vehicles is facilitated.